1. Field of the Invention
The present invention relates to an image forming apparatus and a control method thereof, and, more particularly to an image forming apparatus employing an electrophotographic system and a control method thereof.
2. Description of the Related Art
In an image forming apparatus such as a copying machine employing an electrophotographic system, a process for developing an electrostatic latent image formed on a photoconductive drum using a toner or the like, transferring a toner image developed onto a recording sheet, and then heating and pressing the toner image with a fixing device to fix the toner image on the recording sheet is adopted.
As a structure of the fixing device, a form of using a heating roller having a heater built therein is often adopted. The toner image on the recording sheet is fixed by directly applying heat of the heating roller to the recording sheet or indirectly applying the heat to the recording sheet via a fixing belt or the like.
In order to secure satisfactory fixing performance, it is important to maintain the temperature of the heating roller within a predetermined range.
However, the temperature of the heating roller is low during starting of the image forming apparatus and immediately after the image forming apparatus is returned from a standby mode. Thus, in order to heat the heating roller using the heater to set the temperature of the heating roller within the predetermined temperature range, warm-up for a predetermined time is necessary. In order to reduce this warm-up time, there is a form of using an auxiliary heater in addition to a normal heater (hereinafter referred to as main heater).
For example, in JP-A 2004-253349 and JP-A 2004-310055, there are disclosed techniques for using a commercial power supply as a power supply for a main heater and using an auxiliary power supply, for example, an electric double layer capacitor as a power supply for an auxiliary heater.
In these disclosed techniques, the auxiliary power supply for the auxiliary heater is charged using the commercial power supply when electric power of the commercial power supply is not fed to the main heater. This prevents power consumption of an entire image forming apparatus from exceeding a predetermined range.
In other words, power consumption equal to or larger than a specified value is prevented from occurring by preventing a period of power feeding to the main heater and a period of charging of the auxiliary power supply from overlapping each other.
However, when, for example, a power feeding control signal is used for determination on presence or absence of the power feeding to the main heater, ON and OFF of charging of the auxiliary power supply is controlled according to ON and OFF of the power feeding control signal. In this case, there is a delay time from the time when the power feeding control signal is turned on until electric power is actually supplied to the main heater. Similarly, there is a delay time from the time when the power feeding control signal is turned off until power supply to the main heater actually stops. Because of these delay times, it is likely that the period of power feeding to the main heater and the period of charging of the auxiliary power supply instantaneously overlap each other and power consumption equal to or larger than the specified value occurs.
There is also a method of, for example, detecting the temperature of a main heater with a sensor and determining presence or absence of power feeding to the main heater according to this detected temperature. In this case, a delay also occurs between timing of power feeding to the main heater and a change in the detected temperature of the main heater. For example, since temperature detected by the temperature sensor is low immediately after electric power is supplied to the main heater, it is likely that it is determined by mistake that electric power is not supplied to the main heater to permit charging of an auxiliary power supply. As a result, the power feeding to the main heater and the charging of the auxiliary power supply are simultaneously performed. Electric power equal to or larger than the specified value is temporarily consumed.